You are in a fast powerboat that is capable of a sustained steady speed of 21.0 in still water. On a swift, straight section of a river you travel parallel to the bank of the river. You note that you take 16.1 to go between two trees on the riverbank that are 490 apart.\
Determine its speed
490/16.1 = 30.43
since the boat goes 21.0, the river is going 9.33
You ought to start including the units in your examples. It will make it easier to determine what to multiply/divide by in complex conversions.